1. Field of the Invention
The present invention relates to an apparatus for manufacturing aluminum nitride single crystal, method for manufacturing aluminum nitride single crystal, and aluminum nitride single crystal.
2. Description of the Related Art
An aluminum nitride single crystal type semiconductor has a band gap called 6 eV, which is extremely large among wide gap semiconductors. Thus, an aluminum nitride single crystal is regarded as very promising ingredient for ultra violet LEDs and laser elements. In addition, the grating constant of the aluminum nitride is extremely close to the grating constant of gallium nitride (GaN). Gallium nitride is expected to be a high-voltage high-frequency wave power device. Therefore, an aluminum nitride single crystal type semiconductor is highly regarded as an ingredient for a base member for manufacturing a GaN device.
Examples of manufacturing methods of an aluminum nitride in general include a liquid phase growth method (a flax method), a sublimation method (a sublimation recrystallization growth method), a hydride vapor phase deposition method (a gas phase growth method), and the like. Among these methods, the sublimation method is known as a method which allows a bulk crystal to be formed with a high crystal growth speed. The thickness of the bulk crystal is greater than or equal to a few millimeters. This sublimation method is widely known as a growth method of a single crystal of a carbonized silicon (SiC). Carbonized silicon is regarded as an ingredient of a next generation power semiconductor. Research and development have been performed for the growth of an aluminum nitride (AlN) single crystal by applying the sublimation method.
Here, a sublimation method for the growth of an aluminum nitride in general is described.
FIG. 2 shows an example of a manufacturing device 51 of an aluminum nitride single crystal in general. In FIG. 2, reference numeral 52 represents a crucible 52, reference numeral 53 represents a cap body 53, reference numeral 10 represents a crystal growth furnace, reference numeral 11 represents an aluminum nitride raw material, reference numeral 12 represents a seed crystal, reference numeral 13 represents an aluminum nitride single crystal, reference numeral 15 represents a heating device, reference numeral 16 represents a gas inlet, and reference numeral 17 represents a gas ejection unit.
The crucible 52 is a container created by graphite or tantalum carbide (TaC). An aluminum nitride raw material 11 is stored in the crucible 52. The cap body 53 is placed on an upper surface of the crucible 52. An inner space 14 is created by the cap body 53 and the crucible 52. At a lower surface of the cap body 53, a seed crystal 12 is fixed. The seed crystal 12 is created by an aluminum nitride or a silicon carbide (SiC).
The crucible 52 is fixed inside the crystal growth furnace 10. The crystal growth furnace 10 comprises the heating device 15. At a ceiling part of the crystal growth furnace 10, a gas inlet 16 is formed. The gas inlet 16 introduces nitrogen gas and the like inside the crystal growth furnace 10. At a bottom part of the crystal growth furnace 10, a gas ejection unit 17 is formed. The gas ejection unit 17 ejects gas which was introduced to an interior of the crystal growth furnace 10 such as nitrogen gas, or, gas which was generated at an interior of the crystal growth furnace 10. An interior of the crystal growth furnace 10 is adjusted to be at a predetermined gas pressure by the gas inlet 16 and the gas ejection unit 17.
When the aluminum nitride single crystal 13 is grown, a sublimation is performed by heating the aluminum nitride raw material 11 to approximately 2000° C. with the heating device 15. In this way, a sublimation gas with a composition of aluminum nitride is generated in the inner space 14. Then, the sublimation gas is transported to a location above the seed crystal 12. As a result, the aluminum nitride single crystal 13 is recrystallized above the seed crystal 12 due to the sublimation gas. At this time, in order to facilitate the transportation of the sublimation gas, the temperature of the seed crystal 12 is set to a temperature lower than the temperature of the aluminum nitride raw material 11.
As a sublimation method for manufacturing an aluminum nitride single crystal in general, methods indicated in E. N. Mokhov et al., Journal of Crystal Growth 281 (2005) 93 and C. Hartmann et al., Journal of Crystal Growth 310 (2008) 930 are known. According to these methods, a TaC crucible, a tungsten crucible, a boron nitride (BN) crucible, a graphite crucible, and a graphite crucible whose inner portion is coated with a nitride, are used as a crucible to store an aluminum nitride raw material. When a high-frequency wave heating furnace is used to manufacture an aluminum nitride single crystal, a graphite crucible is used as a heat generator of a heating furnace in general. In this case, an aluminum nitride single crystal is manufactured by placing a crucible for storing raw material inside the graphite crucible.
The sublimation gas generated by a heated aluminum nitride single crystal has a high corrosivity. In particular, a sublimation gas generated by an aluminum nitride single crystal heated to a temperature greater than or equal to 1900° C. has even higher corrosivity as the heating temperature increases. The TaC crucible is considered to be one of the crucibles having the strongest corrosion resistance against a sublimation gas of an aluminum nitride. In this way, it is possible to perform a crystal growth at a temperature greater than or equal to 2000° C. On the other hand, from an industrial standpoint, a crystal growth speed of greater than or equal to 100 μm/h is preferred in a bulk crystal growth. According to the growth of the aluminum nitride single crystal using the sublimation method, it is necessary that the temperature of the seed crystal be greater than or equal to 2000° C. in order to obtain a growth speed of greater than or equal to 100 μm/h. Therefore, the TaC crucible is one of the few crucibles suitable for bulk crystal growth. When this TaC crucible is used, it is possible to obtain an aluminum nitride bulk crystal having a thickness of greater than or equal to a few millimeters.
According to a crystal growth method using a crucible such as the sublimation method, a growing crystal is frequently contaminated as an impurity generated from the ingredients making up the crucible (hereinafter referred to as a crucible ingredient) blends with the growing crystal. In the case of a growth of an aluminum nitride single crystal according to the sublimation method using a TaC crucible, it is disclosed that a carbon of approximately several hundred ppm exists inside the aluminum nitride single crystal (See E. N. Mokhov et al., Journal of Crystal Growth 281 (2005) 93). In this way, when an aluminum nitride single crystal is grown using a crucible comprising a chemical compound comprising carbon (carbide), there is a possibility that carbon is blended inside the crystal inadvertently. Due to such blending of carbon, the phenomena described below are triggered, causing potentially serious problems.
(a) When carbon is blended inside the crystal as a carbon cluster, a crystal growth progresses with each carbon cluster being a nucleus. Thus, polycrystallization is triggered.
(b) Carbon blended into the crystal may become a cause for the generation of a carrier affecting electric conductivity. As a result, a concentration is obtained which is different from a predetermined carrier concentration.
(c) Carbon blended into the crystal generates a lattice defect in the surroundings. As a result, the quality of the crystal is deteriorated.
In fact, a aluminum nitride single crystal obtained by the crystal growth using the TaC crucible described above has a full width at half maximum (FWHM) of greater than or equal to 100 arcsec at a (0002) reflection at an X-ray diffraction locking curve. In this way, the crystallization properties of the aluminum nitride single crystal is not good.
Even when a crucible is used made up of non-carbide type ingredient such as tungsten and the like, there may be instances in which carbon blends with the aluminum nitride single crystal. For example, when a graphite ingredient is placed around the crucible as a heater, the crucible ingredient and a carbon isolated from the heater react with one another in an environment with a temperature greater than or equal to 2000° C. in which crystal growth is performed. As a result, the non-carbide type crucible ingredient is altered to a carbide while the crystal is growing. As a result, similar to the crystal growth using the TaC crucible, there is a possibility that carbon is blended into the aluminum nitride single crystal.
On the other hand, from an industrial standpoint, a crystal growth speed of greater than or equal to 100 μm/h is necessary. Therefore, it is necessary that the temperature of the seed crystal be greater than or equal to 2000° C. However, a TaC crucible and a tungsten crucible are the only crucibles reported to be resistant to the corrosion due to the sublimation gas of the aluminum nitride at a temperature greater than or equal to 2000° C.
The present invention is made in light of these problems. An object of the present invention is to provide an apparatus for manufacturing an aluminum nitride single crystal, which has superior corrosion resistance against a sublimation gas of an aluminum nitride generated while manufacturing an aluminum nitride single crystal, and which can achieve an aluminum nitride single crystal growth velocity of greater than or equal to 100 μm/h.